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The car showed battery light, CEL and all other instrument cluster lights, I pulled over, turned the engine off and it would not start.
The battery was discharged. I took the car home and recharged the battery.
The battery is able to run high beams light for 30+ mins when fully charged. The voltage after overnight charging is 13.2V, after power load - 12.5V.

Now, when I start the engine, the voltage drops to 11.9V, so the alt is not charging.

Further tests. When the key is in On position - battery light is on, the light turns off after the engine is started.

Ground - checked PPF, all ground connections, some took apart, cleaned and put back together. Just to be sure, bypassed the ground altogether by connecting battery (-) to the engine block with jumper cable. That did not help.

Alternator - installed fully, tightened belt, the pulley is confirmed spinning when the engine is running.

Voltage.

When Off

White wire - 12.57V

White/black - 0V

White/green - 12.56

When key is in On, car not running

White wire - 12.24V

White/black - 0.68V

White/green - 12.25

When the car is running

White wire - 12.01V

White/black - 10.95V

White/green - 11.99V, then drops to 10.96V

Something seems to seem odd with these numbers in red, why is 1V drop when running? It should match the battery voltage...
I found this forum after watching this video

, but my symptoms are a bit different. I do not have a full power loss in either of the regulator circuits.

Another observation. If the car is running at the fully charged battery that has a battery charger attached to it and supplying 12V/5A, I can see the battery voltage start going up to 14.5V.

Where can I get the wiring diagrams to run the diagnostics as it was shown on the video?

BTW, the car has a non-working power antenna, also some other minor intermittent electric problems.

I see 4 possible causes some simple checks with a voltmeter and tungsten test light first but I think you will end up with a rebuilt alternator. The early MX5/Miata/Enos was fitted with an alternator that is a bit undersized by modern standards however it is a very simple system.

First test is unplug the 3 pin connector on the alternator and check for battery voltage at each of the three terminals in the plug. Both the White-Green and the White should be live with the ignition on ---- if not check the 80a and 30a fusse and for wiring damage or connector corrosion between the plug and the fuses. Next with it still unplugged and ignition on test for near battery voltage on the black-white wire --- I would also do this test with tungsten test light --- which should glow dimly.
If these tests are okay reconnect the plug and put the voltmeter across the battery terminals , back probe the black and white wire at the alternator and supply it with battery voltage through the test light. With the engine running if the alternator is okay it should start charging

Sorry I misunderstood your intro post. You're clearly on top of things, as far as testing!

At the risk of differing from Andy (who is a sharp cookie), I think you're done. You need an alternator. It has everything it needs to do it's job, and still isn't. Your testing shows a working light circuit, a working regulator power circuit, and no opens in the main power or ground to the alternator.

The only part that might suggest an issue is the change in voltage you saw on the white/green wire. A drop of 1V over a Main Relay circuit isn't great, obviously. But the 11V you saw should be enough to run the alternator regulator. It doesn't help that the alternator isn't charging during these readings. If you still have doubts, you can take a jumper from the white cable to the white/green wire and see if it makes any difference. If it does, we can dig deeper into the Main Relay circuit. If it doesn't, you still need an alternator.

We ain;t differing Tyler ---- foobared alternator is most likely cause. Not sure if these units are easy to fix some Denso units can have the regulator and brush box changed in about 10 minutes with alternator on still he car

As Andy said, I think the best course.of action if so checks out is to supply the alternstor with a signal. If it does not start charging, you likely need a new alternator. If it does, you have a circuit problem.

I did not test it today, came back home when it's already dark. But I remember testing it a month ago, with the connector unplugged. These measurements are on the terminals of the wire connector. It actually is a 2-wire connector, not 3-wire (G/W and B/W wires, the thick white wire has it's own lug nut attachment to the alternator, despite running in the same harness)

So again, the voltages are taken at the terminal of 2-wire plug, (not the alternator coupling terminals). Also, the voltage on the solid wire was taken while it is attached to the alternator. The plug is disconnected from the alternator and measured, white wire is connected to the alternator while measured.

With key switch off:

white wire -- 12.67V

white/black wire -- 0V

white/green wire -- approximately 12.68V

With key switch on:

white wire -- 12.34V

white/black wire -- 11.97V

white/green wire -- 12.34V

The number in red looks suspicious to me, my understanding it should be around 1V when the engine is not running, but I am not confident if this "around 1V" is obtained from the B/W wire while it is connected or disconnected.
When the plug is connected, I get a bit less than 1V on the B/W wire (key "on", engine off), through the sewing needle with which I tapped the wire.
In a day or two I should get a more solid connectors to tap into those wires. This is it for now...

That's my third one... Got one from acdelco, not charging. This one Mazda rebuilt, made by Mitsubishi. New in box from ebay. Not charging. Not to mention the one that died and started my whole journey. That one lastx for 2 years, but itself was a third replacement for the one that came with the car. My Miata seems to be eating them.
You might be right though. I'll wait until testing with tungsten wire circuit tester and only then will give up on it.

It seems to be an issue on these early MX5 cars, probably the 60 or 70 amp alternator is supplying near its max output on a high revving engine. If it were mine I would check on the one make sites for an easy upgrade to say a 90 or 105 amp unit.
It is a fairly standard 3 wire alternator complete with regulator, if you can find an alternator that will accept the v pulley and fit the mounting brackets it should work.

Just did it. No change and the system voltage does not go up. So, you think alternator is dead?

There was something else that helped (I was not able to monitor the alternator sounds since I had to go inside the car to start it so I'm not sure if this really is an alternator waking up. ut if not, then I have no explanation for what I am writing below).
If the battery is fully charged (overnight on 5A current) and shows 13.2 volts,
and also, if I put 30A input on the battery charger and keep it connected to the battery while the car is running,
then the alternator seems to wake up and the voltage goes up to ~14.1-14.5V depends on revs (and this means yes, the voltage starts responding to the engine revs).
If I unplug the charger, nothing changes immediately, but in around 3-5 mins the voltage goes down to 12.2-12.3V range and and does not elevate in response to engine revs.

Taking the alternator apart was not hard.
What would be the way to test the voage regulator and/or diodes? I want to understand what failed in my alternator and that I'm placing the working replacement.
These are the pictures of the stator and electronics inside and new part that came in.

The rectifying diodes you can check with the diode function of your dvom. Put one meter probe on the positive outlet post (where your "thick" white cable was connected and the other lead to the diodes themselves, one at a time. There should be flow into one direction and no flow when you reverse the probes.
Any diode that allows current to flow in both or no direction is at fault and you will have to replace the rectifier as a unit.

Concerning the regulator there is no practical way I know of to "test" it easily on the bench, iout of the car.If the brushes and the spring tension/collector slip rings are fine then you just may want to put a new one in...

With the rest of the alternator confirmed healthy you can check the regulator IN the car by applying varies voltage levels to the control wire. The alternators output should respond if it is sound.

An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.

I wasn't able to clearly find both poles of the diodes to test. One was obviously a soldered spot with the coil end. There were no other contact spots, other than the radiator into which they were pressed into.

However, I think I found something that looks like potential culprit.
On the new regulator, there's continuity of the top most brush to the metal contact seen on the low left side, see the picture below.
In the non-working alternator, the same path shows no conductivity.
Which way is it suppose to be?
I am hesitating because 5he original voltage regulator looks more advanced than the replacement one (check the pictures in the previous post).
And there are other soldering spots which purpose I cannot understand or guess.
Should this circuit be conducting, like on the replacement part or is it going live when I supply the regulating current?